An antiquities museum in Syria's rebel-held province of Idlib said to house one of the world's oldest dictionaries reopened on Monday after five years, an AFP correspondent said.

An explosion in a residential building thought to be storing weapons and ammunition in Syria's rebel-held Idlib killed at least 39 people including 12 children on Sunday, a war monitor said.

The announcement came with a photo of the first lady in loose clothing and trainers, sitting on a chair in a hospital room with an intravenous drip inserted in her left hand.

The head of a Syrian research facility that Western countries say was part of a chemical weapons programme was killed when his car was blown up, the pro-Syrian government newspaper al-Watan said on Sunday.

Government forces are on the brink of seizing the last part of southwestern Syria in insurgent hands, the Syrian Observatory for Human Rights said on Monday.

A Syrian Kurdish group said on Saturday it had decided with the government to "chart a roadmap to a democratic and decentralised Syria", but there was no immediate confirmation from Damascus.

A string of suicide blasts and raids claimed by the Islamic State group killed more than 180 people in southern Syria on Wednesday, in one of the jihadists' deadliest ever assaults in the country.

Israel shot down a Syrian fighter jet with surface-to-air missiles on Tuesday after the plane infiltrated airspace it controls, the military said, in a rare incident that could provoke tensions.

In Lebanon's eastern border town of Arsal, men, women and children of all ages piled into cars, minivans and tractors.

Russian and Syrian jets stepped up their bombing of an Islamic State bastion along the Jordan-Israel border in southwestern Syria, as the militants pushed into areas abandoned by other rebel groups, diplomatic and opposition sources said.

The Israeli military said that it had completed "a humanitarian effort to rescue members of a Syrian civil organisation and their families".

The Syrian army and its allies made advances in the southwest that bring it closer to the Israeli-occupied Golan Heights border, state television and rebels said on Saturday.